This paper examines the conditions under which annual hours (AH) are likely to succeed or fail and the role of workplace partnership in delivering mutual gains. We explore two case studies, in one company with a positive experience and in a second where AH were regarded as a failed initiative. The case studies are constructed from interviews with trade union and management representatives in the companies involved; from secondary sources and from a worker survey. The findings echo previous research that AH can deliver mutual gains in both the presence and absence of workplace partnership (Author and Author, 2016) and that delivery of real mutual gains is the key driver of the long term viability of AH. However, the balance of mutual gains is subject to change and is strongly influenced by structural factors determining the suitability of AH to the particular enterprise.